Burning of fuel to run an engine in a controlled environment is called combustion and since the combustion happens inside the cylinder, these engines are called internal combustion engines. The most common internal combustion engine type is gasoline / petrol powered. Others include those fueled by diesel, hydrogen, methane, propane, etc. Engines typically can only run on one type of fuel and require adaptations to adjust the air/fuel ratio or mix to use other fuels. Burning fuel in a controlled way inside the engine was a problem that was solved in the late 19th century with the advent of the 4-stroke or Otto cycle (named after the inventor of this concept).

2 STROKE ENGINES

The two-stroke engine is simple in construction. The cylinder has no intake or exhaust valves. Intake and exhaust are accomplished by means of ports - special holes cut into the cylinder wall which allow fuel-air mixture to enter and exhaust to exit the engine. Air-fuel mixture is drawn into the crankcase from the carburetor.

When the piston is forced down (One Stroke), the exhaust port is uncovered first for the hot exhaust gases to leave the cylinder, with the piston in it's downward most position the crankcase becomes pressurized the intake port into the cylinder is uncovered allowing pressurized air-fuel mixture to enter the chamber. As the piston begins to move up (Stroke Two), the ports are closed off, and air-fuel mixture gets compressed by the rising piston in the cylinder and the mixture is ignited.

Since the two stroke engine fires on every revolution of the crankshaft, a two stroke engine is usually more powerful than a four stroke engine of equivalent size. This, coupled with their lighter, simpler construction, makes the two stroke engine popular in chainsaws, line trimmers, outboard motors, snowmobiles, jet-skis, light motorcycles, and model airplanes.Unfortunately, most two stroke engines are inefficient and are terrible polluters due to the amount of unspent fuel that escapes through the exhaust port.

4 Stroke Engines

The four-stroke cycle is such a good concept that almost all car engines use it. Unlike two-stroke engines (used in small bikes) in which combustion takes place every alternate stroke, in a four-stroke engine, combustion is more efficient and takes place once in four strokes. In the first stroke, fuel-air mixture is sucked into a cylinder through an inlet valve(s). During the second stroke, the piston in the cylinder moves upwards and compresses the air-fuel mixture into a very small space that is the combustion chamber. The ignition of the fuel is precisely timed when the piston reaches the top of its stroke and the valves are closed. This ignition is a mini-explosion, which forces the piston down and this is the third stroke or ‘power’ stroke. During the last stroke, the exhaust valves and piston move upwards to expel the burnt gases. The cycle is then repeated again with the intake or suction stroke.

Combustion Cycle

Intake stroke

The piston starts at the top, the intake valve opens, and the piston moves down to let the engine draw in a cylinder-full of a mixture of air and petrol. The lowest most position of the piston is know as BOTTOM DEAD CENTRE (BDC). This stroke is known as intake stroke.

Compression stroke

The piston moves back up to compress the air and petrol mixture to about one-tenth of its original volume. Compression makes the explosion more powerful. The topmost position of teh piston is known as TOP DEAD CENTRE (TDC).

Combustion stroke

When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the compressed fuel & air mixture.

Exhaust stroke

The burn mixture expand he gasses and pushes the psiton down to its BDC. Exhaust valve opens and the piston returns to TDC pushing the burned exhaust gas from the explosion out of the cylinder. And then the process repeats itself by taking in another charge of air and fuel.

The rotational motion of an internal combustion engine is converted to linear (straight line) motion crankshaft.

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